1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire suitable for use in truck, bus, industrial vehicle, construction vehicle and the like. More particularly it relates to a heavy duty pneumatic radial tire having a bead portion durability improved by effectively controlling both a separation failure created in a turnup portion of a radial carcass ply and a wearing and a surface cracking of an outer surface of a bead portion due to friction with a rim flange, which tend to be caused together with repetitive falling-down deformation of the bead portion during the running of the tire under loading.
2. Description of Related Art
As means for improving the bead portion durability in the pneumatic tire, it is useful to arrange a reinforcing member such as a nylon chafer, a wire chafer or the like at an outer peripheral side of a turnup portion of a carcass ply around a bead core, particularly at a side of the turnup portion opposing to that portion of a rim flange which is apt to cause troubles in the bead portion.
However, when plural reinforcing members are arranged for improving tire durability, the increase of weight and the like is caused but also layers resulting in the occurrence of separation failure is newly formed and finally the bead portion durability can not effectively be improved.
As another means for improving the bead portion durability, it is useful to prevent the falling-down deformation of the bead portion in the inflation under an internal pressure or in the loading or control the shearing deformation created in a circumferential direction at a stepping-in or kicking-out side in the loading by modifying the arrangement of rigidity or the like through the rationalization of so-called carcass line. However, the improvement of the bead portion durability only by such a means has a natural limit considering the present state that the service condition of the tire becomes more severer.
When a load is applied to the tire, a side portion of the tire located just under the loading produces a deflection deformation outward in the widthwise direction of the tire and inward in the radial direction thereof, and the falling-down deformation of the bead portion is caused accompanied with such a deformation. The falling-down deformation of the bead portion is caused to rotate about the bead core as a fulcrum, whereby a member sandwiched between the turnup portion of the carcass ply and the rim flange portion (concretely, a reinforcing rubber located between a rubber chafer constituting the outer surface of the bead portion and the turnup portion of the carcass ply) is largely compressed. The compressed member flowingly moves along an upper portion of the rim flange (i.e. toward the outside in the radial direction of the tire), and hence the shearing deformation is repeatedly caused between the compressed member and cords embedded in the turnup portion of the carcass ply during running of the tire under loading. Such a shearing deformation results in the occurrence of the separation failure at an interface between the cord embedded in the turnup portion and an outer rubber (a coating rubber for the carcass ply) or in rubber near the cord.
For example, under a situation that the shearing deformation is repeatedly produced between the turnup portion of the carcass ply and the reinforcing rubber adhered through curing to the outer surface of the turnup portion, in case of the tire adopting so-called low-turnup structure that the turnup end of the carcass ply is located near to the bead portion of the tire, the stress tends to concentrate in the position of the turnup end and the separation failure is apt to be caused at this position.
For this end, in case of the tires for construction vehicle having a large deflection ratio under loading, it is generally known to adopt a so-called high turnup structure that the turnup end of the carcass ply is arranged at a position not subjected to an influence through the falling-down deformation of the bead portion, that is, at a position of the sidewall portion or at a position reaching to the belt. In the tire adopting the above construction, the separation failure at the turnup end of the carcass ply due to the falling-down deformation of the bead portion hardly occurs.
Even in the latter tire, however, there is a tendency of causing the separation failure between the cord embedded in the turnup portion and the outer rubber (coating rubber for carcass ply) accompanied with the shearing deformation in the turnup portion of the carcass ply located at the bead portion of the tire.